1. Field of the Invention
The present invention relates to means for preventing color signals from exceeding upper and lower threshold values.
2. Description of the Prior Art
U.S. Pat. No. 3,848,856 describes an apparatus in which a raster scan scans a color image for example a color slide or finished color extraction and supplies a set of color component signals which are subjected to an electronic color correction and then supplied to a video display receiver for observation and control. The color component signals represent the color dosage values for the individual print colors magenta, cyan, yellow and black and are to be corrected with an apparatus having a level indicator under viewing control. So that color images will also be formed on the video receiver, the impression is produced by an image printed with the corrected color component signals with the aid of the corresponding color extractions and a print reproducing computer is provided between the correction stage and the video receiver which takes into consideration the fixed parameters of the respective printing method for display. In such apparatus, the color correction is accomplished in selected local areas and made visible. The respective color component signals are measured and supplied to a person skilled in the art in order to allow him to correspondingly rework by hand the selected local areas of the color extraction slides or plates. These corrections are carried out either by etching or by photographing exposure with corresponding masks.
Such apparatus has the advantage that the corrections can be visibly observed, however, it is a disadvantage that the actual correction of the color extractions must be accomplished by hand which is time consuming and does not assure that the corrections in the color extraction precisely correspond with the corrections appearing on the video receiver as this depends exclusively upon the skill of the operator making the corrections.
Equipment is also known which allows the production of corrected color extractions automatically. However, the adjustment of the correction must be undertaken by hand and by densitometer measuring of various image points of the original and subsequently evaluation by a person skilled in the art who sets the color correction regulator. Such equipment has the disadvantage that the correction also depends upon the skill of the operating personnel and can only be controlled in the finished color extraction or in the printed product. To provide such equipment with video display equipment, display on a video display means can only be realized with very high storage expenses as the data which are supplied by a flat belt scanner or a spiral drum scanner or a drum scanner with intermittent feed are not suited for producing a still picture because such pictures occur only once and a drum or belt recording apparatus is controlled on a line.